1. Field of the Invention
This invention relates generally to compact, light weight printheads and, more particularly, to integral Organic Light Emitting Diode (OLED) fiber optic printheads.
2. Background
Light emitting diodes (LED) have been used for exposing photosensitive materials such as photographic film or photographic paper or photocopying receptors. The light emitting diodes are usually arranged in a linear array or a number of linear arrays and means are provided for a relative displacement of the photosensitive materials in relation to the array. In this manner, the material is scanned past the array and an area is exposed thereby creating an image.
The light emitted from LEDs diverges quickly and thus reduces the exposing intensity and increases the exposing area. This can lead to a reduction in sharpness of the exposed image and to the possibility of undesired exposure of adjacent areas. The first of these problems is known as reduced pixel sharpness and the second is known as crosstalk. To avoid these difficulties, optical systems are utilized to transmit the light from the LEDs to the photosensitive material without significant divergence. While this approach results in an acceptable printing system, such systems have their size defined by the optical systems and therefore are not as compact as would be desired for a portable print system.
Organic Light Emitting Diodes (OLED), which have been recently developed, (See, for example, the article by S. Forrest, P. Burrows, and M. Thompson, xe2x80x9cThe dawn of organic electronicsxe2x80x9d, IEEE Spectrum, Vol. 37, No, 8, pp. 29-34. August 2000) hold a promise of ease of fabrication and low cost and low power consumption. A recent publication (Y. Tsuruoka et. al., xe2x80x9cApplication of Organic electroluminescent Device to Color Print Headxe2x80x9d, SID 2000 Digest, pp. 978-981), describes a print head utilizing OLEDs. The printhead described in this publication is comprised of discrete OLEDs, color filters and optical elements and therefore is not as compact as desired. Also, the presence of discrete optical elements requires considerations of alignment which have an impact on manufacturability and cost.
While it would be preferable to dispense with the use of optical elements (see related applications Ser. No. 09/749,346 filed on Dec. 27, 2000 and Ser. No. 09/745,042 filed on Dec. 20, 2000), there are some cases of interest where obtaining the best printing conditions requires using optical elements. Among the proposed optical elements that have been proposed by others are arrays of graded index lenses and arrays of graded index optical fibers. Both of these proposed solution (see for example, U.S. Pat. No. 4,447,126, entitled xe2x80x9cUniformly Intense Imaging by Close Packed Lens Arrayxe2x80x9d, by P. Heidrich et al, and U.S. Pat. No. 4,715,682, entitled xe2x80x9cMount for Imaging Lens Array on Optical Printheadxe2x80x9d, by K. Koek et al.) require considerations of alignment and assembly. An Integral Fiber Optic printhead which utilizes electrical connection means to connect the light emitting diodes to conductive lines on the substrate has been described in U.S. Pat. No. 4,921,316 (Fantone et al., Integral Fiber Optic Printhead). The light emitting diodes used in present printers (see for example, Shimizu et al., LED Arrays, Print Head, and Electrophotographic Printer, U.S. Pat. No. 6,064,418, May 16, 2000) emit radiation from the surface of a p-n junction (constitute edge emitters) and are typically mounted on a printed circuit board. These characteristics of the LEDs used in previous printers impose constraints on manufacturability and the ability to optimize performance.
It is the primary object of this invention to provide an integral printhead which is compact, light weight, requires minimal alignment and utilizes Organic Light Emitting Diodes (OLED). It is a further object of this invention to provide an integral printhead which provides the necessary pixel sharpness while avoiding crosstalk and which utilizes Organic Light Emitting Diodes (OLED). Other objects of this invention will become apparent hereinafter.
To provide a printhead that is light weight and compact, which is the primary object of this invention, an OLED structure is disposed onto a fiber optic faceplate substrate. The fiber optic faceplate substrate has a substantially planar light receiving surface oppositely spaced apart with respect to a substantially planar light emitting surface. The fiber optic faceplate comprises a plurality of individual glass fibers which are stacked together, pressed and heated under pressure to form a uniform structure with a plurality of light transmitting passages extending between the light receiving and light emitting surfaces. The OLED structure is placed on the light receiving surface of the fiber optic faceplate substrate. The OLEDs emit radiation in one of at least three separate wavelength ranges. To provide an integral printhead that provides the necessary pixel sharpness while avoiding crosstalk, the printhead is designed for direct printing with the desired pixel sharpness and reduced crosstalk.
In one embodiment, the OLED structure comprises at least one elongated array of individually addressable Organic Light Emitting Diode (OLED) elements deposited onto the fiber optic faceplate substrate. Two possible different arrangements for the printhead are disclosed. In one arrangement, each OLED array in the printhead comprises at least one of a plurality of triplets of OLED elements, and each element in each said triplet being capable of emitting radiation in a distinct wavelength range different from the distinct wavelength range of the other elements in the same triplet. In the second arrangement, the printhead comprises at least one of a plurality of triplets of elongated arrays of individually addressable Organic Light Emitting Diode (OLED) elements. Each array in the triplet is aligned in substantially parallel relation to any other array in the triplet. Each array in each triplet has elements that are capable of emitting radiation in a distinct wavelength range different from the distinct wavelength range of the other two arrays in the triplet.
In second embodiment, the OLED structure comprises a substrate having a planar first surface opposite to a planar second surface and at least one elongated array of individually addressable Organic Light Emitting Diode (OLED) elements, the at least one array of OLED elements being disposed on the second surface of the OLED structure substrate. A substantially transparent layer is deposited onto the at least one elongated array of individually addressable Organic Light Emitting Diode (OLED) elements. The substantially transparent layer has a light receiving surface in effective light transmission relation to the OLED elements, the light receiving surface being located opposite to a light emitting surface. The OLED structure is disposed on and mechanically coupled to fiber optic faceplate. Again, the same two alternative arrangements previously disclosed are applicable for this embodiment.
The parameters including the distance between OLED elements, the characteristic dimension of the OLED elements, the distance between the light emitting surface of the fiber optic faceplate substrate and the photosensitive material, the numerical aperture of the optical fibers, are selected to optimize the exposure of the photosensitive material at a given pixel area corresponding to a given OLED element, due to the light intensity from the elements of the array which are adjacent to the given OLED element and from the given OLED element. An exposure is optimized if the Subjective Quality Factor (SQF) of the resulting pixel is as close to 100 as possible and if the intersection of the normalized intensity profile produced by an adjacent color filter array element at given pixel locations with the normalized intensity profile produced by the corresponding color filter array element is as close to 0.5 as possible.
The printheads of this invention can be used to expose the entire gamut of photosensitive materials, for example, silver halide film, photosensitive paper, dry silver, photocopyng receptor material, imageable materials comprised of dyes, acid amplifiers and other photosensitive compounds.
These embodiments provide printheads that are light weight and compact, where an OLED structure is disposed on a fiber optic faceplate substrate. The printheads are designed for direct quasi-contact printing with the desired pixel sharpness and reduced crosstalk. By virtue of their compactness and their light weight, as well as the low power requirements of OLED elements, the printheads of this invention enable the construction of portable printing devices for the mobile data environment.